Unsaturated Ni^II Centers Mediated the Coordination Activation of Benzylamine for Enhancing Photocatalytic Activity over Ultrathin Ni MOF-74 Nanosheets

Abstract

Creating accessible unsaturated active sites in metal-organic frameworks (MOFs) holds great promise for developing highly efficient catalysts. Herein, ultrathin Ni MOF-74 nanosheets (NMNs) with high-density coordinatively unsaturated Ni^II centers are prepared as a photocatalyst. The results of in situ ATR-IR, Raman, UV-vis DRS, and XPS suggest that abundant Ni^II centers can act as the active sites for boosting benzylamine (BA) activation via forming -Ni-NH₂- coordination intermediates. The generation of coordination intermediates assists the transfer of photo-generated holes to BA molecules for producing BA cation free radicals, better impelling the breaking of N-H bonds and the photooxidation of BA molecules. The photo-generated electrons further activate O₂ molecules to O₂^•- radicals for triggering the reaction. The experiments reveal that the coordination activation of BA molecules may be a rate-determining step on NMNs rather than the adsorption and activation of O₂ molecules. Moreover, NMNs possess a better ability for the separation of photo-generated carriers in comparison with bulk Ni MOF-74 (NMBs). As a result, NMNs achieve a kinetic rate constant of 0.538 h^-1 for the photocatalytic oxidative coupling of BA under visible light, about 50 times higher than that of NMBs (0.0011 h^-1). Finally, a probable synergetic catalytic mechanism with coordination activation and photocatalysis is discussed on a molecular level. This study not only highlights the importance of coordination activation for heterogeneous photocatalysis but also affords an inspiration for building ultrathin MOF nanosheets.